1. Field of the Invention
The present invention relates to a booster circuit. More particularly, this invention relates to a charge pump type booster circuit.
2. Description of Related Art
Generally, a booster circuit is required to keep an output voltage constant. However, in a charge pump type booster circuit, the output voltage drops in a relatively short period of time when a value of a current flowing to a load becomes larger. Note that the load is connected to the booster circuit in general.
A related technique disclosed in a Japanese Unexamined Patent Application Publication No. 8-149802 is shown in FIG. 9. As shown in FIG. 9, a booster circuit 100 has two booster units 110, 120. Control signals A and B are input to the booster unit 110. The control signals A and B are input to the booster unit 120 after inverted by buffers, which are connected to input terminals of the booster unit 120. A capacitor C100 is charged by the booster unit 120 when the control signal A is high (H) and the control signal B is low (L). The capacitor C100 is charged by the booster unit 110 when the control signal A is low (L) and the control signal B is high (H). That is, the capacitor C100 is charged by the booster unit 110 and the booster unit 120 alternately along with the time axis.
It is common in the charge pump type booster circuit to use external capacitor parts, which are provided separately from an IC (Integrated Circuit) chip including the charge pump type booster circuit. Because the charge pump type booster circuit requires a large capacitance value, it is often not enough for an integrated capacitor to satisfy the required capacitance value.
In this case, an increase in a number of the capacitors results in an increase of manufacturing cost of the circuit. When a plurality of booster units as shown in FIG. 9 are used so as to suppress ripples of the output voltage, manufacturing cost of the circuit raises accordingly and it becomes difficult to suppress the cost up of the circuit.
As explained above, it was difficult to suppress a total number of external capacitors so as to keep the output voltage constant.